1. Field of the Invention
This invention relates to an automotive driveline wherein a friction clutch is used in place of a hydraulic torque converter to driveably connect the engine shaft to the input shaft of an automatic transmission. More particularly, the present invention relates to a system for controlling operation of the clutch in a neutral-idle condition, during acceleration of the vehicle and during certain conditions when reduction of driveline noise, vibration and harshness is required.
2. Description of the Prior Art
Hydraulically actuated, mechanically driven automatic transmissions generally include a torque converter having a turbine driven hydrodynamically from the impeller, the turbine transmitting power to a gearset to produce the reverse drive and forward drive ratios. When any forward or reverse gear is selected, and though the vehicle is coasting or standing at idle, the engine transmits torque through the torque converter, the clutches and gears to the drive wheels. Although the accelerator pedal is released, this causes the vehicle to creep forward against the action of the service brake which must be activated to keep the vehicle stationary. Normally, the gear selector must be moved to the neutral position to disengage the forward gear.
It is preferable that the transmission be automatically shifted to the neutral position from the forward gear position whenever engine power is not required during the idle and coasting condition. The engine idle system then can be calibrated to improve fuel economy by reducing the torque transmitted through the automatic transmission.
The output torque transmitted by the torque converter is a maximum when the device is started, i.e., when its output shaft is stalled, and varies substantially proportionally with the square of the impeller speed. When starting from rest, the efficiency of the torque converter is zero. As the turbine gains speed, its efficiency increases to a maximum, usually in the order of 85 to 90 percent, when the ratio of the turbine speed to the impeller speed is roughly 65 percent. As turbine speed increases toward 80 percent of the impeller speed, the efficiency declines from its maximum. Therefore, over a substantial portion of the operating range of the torque converter its efficiency is below 80 percent; at speed ratios less than 40 percent, its efficiency averages about 40 percent. For a vehicle operating at highway speed, when the output torque of the torque converter drops to the value of the input torque, the device operates as a hydraulic coupling and its efficiency ranges between 80 percent and 95 percent. It is preferred that the characteristics of the torque converter that permit transmission of power silently and smoothly without shock be realized in a device whose efficiency is at least in the upper range of the torque converter efficiency or preferably greater than this range.